We’re going away on Thursday for a few sunny days near the Tropic of Cancer. I’m a bit sheepish, because we keep resolving never to get into another airplane. So I’ll make a mystery of it – make you a small astronomical-cum-geographical riddle to solve, like the “Forbidden Island” one of three years ago.
Think of the Greek capital letter Pi, which looks like two legs with a bar across the top. Slant it so that it tips to the left. There is a constellation shaped like that – two parallel lines, sloping from northeast to southwest, roughly from declination 30 down to 20 north.
There is also a pair of long thin geographical features on our planet, parallel to each other and sloping the same way, from northwest to southeast, also roughly from latitude 30 to 20 north.
(Northeast to southwest, northwest to southeast – are those the same directions of slope? Yes, because you look up at the sky and down at the Earth.)
So when the constellation passes overhead of the geographical formation, they overlie, or mirror, each other. If you could look down through the constellation, you’d see it printed over the geographical pair. That image doesn’t work very well, since the stars are at such huge and various distances, so let’s try another: it’s as if the piece of geography is a rug on a floor, reflected in a mirror in the ceiling.
As the constellations pass overhead, this overlying happens at about sidereal time 7 hours, which, at this time of year, is an hour or so after local midnight.
We won’t be flying to the constellation, but to a bay at the southern end of the geographical formation.
And another clue – and another reason, sort of, for going there – is that this bay is a few miles along the same bit of mountainous coast from a similar though smaller bay from which I and a company of others watched the great eclipse of 1991. For some, that may make the geographical part of the puzzle too easy.
And a clue that may do the same for the astronomical part: On the penultimate night before we come back, we should be able to watch, from this beach, the meteors that fall all night from this constellation. Sparks falling from the mirror to the rug, about vertically.
Meanwhile, here is the Moon rising this evening.
It was Full yesterday, Sunday Dec. 3, at about 16 Universal Time (10 AM Central Standard Time), and at perigee today at 9 UT (3 AM), and as perigee was so close to the Full moment it was the second-nearest perigee of the year (after that of May 26), and you may experience very high tides, though not if you are in the Central time zone.